The present invention relates to automatic time-based measurements, e.g. Time of Flight (TOF) measurements or other time-related measurements, such as performed by LIDARs (light detection and ranging devices), robots, self-driving cars, drones, phase-locked loops, mass spectrometers, and other automated equipment.
Time-based measurements are widely used to measure distances, signal phase differences, and masses. For example, a robot can determine the distance to an object by emitting a light beam and measuring the time traveled by the light to the object and back. A phase-locked loop (PLL) can determine the phase difference between two signals by measuring the time interval between the signals crossing a threshold value. A mass spectrometer can determine composition of a sample by measuring the time that it takes a sample's ionized molecules to travel a predetermined distance in an electric field.
FIG. 1 illustrates a time measurement device 110, called “time-to-digital converter” (TDC). TDC 110 accepts a Start signal and a Stop signal. The Start signal is asserted at some time tstart when the measured time begins, e.g. when a robot emits a light beam. The Stop signal is asserted at a later time tstop when the measured time ends, e.g. the robot detects the reflected light. TDC 110 outputs a digital signalT=tstop−tstart 
which represents the duration being measured.
A multi-stop TDC 110 (FIG. 2) determines durations of multiple events, all of which start at a time tstart. Different events end at possibly different times tstop_1, tstop_2, etc., as indicated by signals Stop_1, Stop_2, etc. For each event j, the TDC outputs the digital signal Tj=tstop_j−tstart representing the event duration.
The multi-stop TDC can be implemented as a number of single-stop TDCs Each single-stop TDC could accept the Start signal and a corresponding Stop signal Stop_j, and provide the digital Tj signal. However, a smaller and simpler multi-stop TDC is desired.